Fix #34779: Channels disappear from multilayer exr sequence

Issue was caused by delayed or missing image user frame
number update, which lead to image loading failure in
cases node is updating from image signal callback.

Solved in a way that file from image datablock is used
for sockets detection instead of loading image for
current frame.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55624 954f8c5b-7b00-dc11-b283-0030488c597c